Two-way hybrid fiber-coaxial (HFC) networks are shared bi-directional networks with point-to-multipoint transmission in the downstream direction, and multipoint-to-point transmission in the upstream direction. Signals are distributed via a fiber optic connection from a head-end to a node that converts the optical signal to an electrical signal, and then distributes the signals to residences via a tree and branch coaxial cable distribution network. At the subscriber side, terminal equipment supports the delivery of cable services (video, data and voice services) to subscribers, via cable modems. Data and voice services are supported by cable modems and communication gateways, respectively, which require the use of an upstream signal path. The network uses a fiber optic upstream signal path from the node to the head-end. A return band is used to support transmissions from devices at subscribers' premises back to the head-end. In such networks, many cable modems may compete for communication bandwidth in both the upstream and downstream directions.
A cable modem generally uses standardized communication protocol based on the Data over Cable System Interface Specification (DOCSIS®) to access data services through the cable network, by using the downstream path to indicate exactly when each modem is permitted to transmit in the upstream direction. DOCSIS outlines issues related to system maintenance, operation and network communications.
The DOCSIS utilizes two primary data transmission elements: (a) Cable Modem Termination System (CMTS) located in specified nodes on the HFC network for distributing data to end-of-line subscribers, and (b) a Cable Modem (CM) residing at subscriber's premise. Additional network elements may be placed inside the cable network to support service delivery or to monitor service quality. In general, a user connects a cable modem to the TV outlet for his or her cable TV, and the cable TV operator connects a cable modem termination system (“CMTS”) in the operator's headend and/or hub. Subscribers send data from their digital devices, such as personal computers (PC), VoIP phone, Video IP device, etc, into the cable modem, which then relays the data to the CMTS, which in turn relays the information to an appropriate network element. Information destined to the subscriber digital device is provided from the network to the CMTS, which in turn relays the information to the CM. The CM in turn relays the information to the subscriber's digital device. The communication direction from the CMTS to the CM is referred to as the downstream direction, while the communication direction from the CM to the CMTS is referred to as the upstream direction.
The fourth version of DOCSIS®, DOCIS® 3.0, was released in August 2006; it specifies support for channel bonding that provides a flexible way to significantly increase transmissions speeds, both upstream and downstream, and introduces support for Internet Protocol version 6 (IPv6) and support for IPTV. The term ‘channel bonding’ means a process that combines data received on multiple independent channels into one higher-speed data stream.
DOCSIS® 3.0 physical layer specification provides for a normal downstream operating range from 50 MHz to 1002 MHz, with either 6 MHz or 8 MHz spacing for downstream channels, which utilize 64-QAM or 256-QAM modulation format. The upstream operating frequency range may be between 5 and 42 MHz, or 5 to 85 MHz. The upstream channel widths may be between 200 kHz and 6.4 MHz, with the upstream data modulated with either QPSK, 16-QAM, 32-QAM, 64-QAM or 128-QAM.
In order to increase downstream and upstream throughput, DOCSIS 3.0 compliant CMs have the ability to flexibly bond at least 4 channels in the downstream and at least 4 channels in the upstream. Channel bonding can be implemented independently on upstream channels or downstream channels. DOCSIS 3.0 enables a total upstream throughput of 30.72 Mbit/s per 6.4 MHz channel using 64-QAM at 5.12 Msps symbol rate, and a downstream throughput with 256-QAM of up to 42.88 Mbit/s per 6 MHz channel, or 55.62 Mbit/s per 8 MHz channel for EuroDOCSIS.
With the channel bonding, the maximum throughput increases in proportion to the number of channels being bonded. Future versions of DOCSIS will likely require that cable modems bond an increasing number downstream and upstream channels.
One technical challenge in operating a network having a bidirectional communication path on a shared medium between the headend and each remote point is maintaining network integrity for upstream and downstream signals. Noise and other undesirable energy originating at one remote point or at any point along the return path from that remote point can impair network communications for all remote points in the network. Similarly, where noise and undesirable energy from one remote point is combined with noise and or other RF impairments from other remote points in the network, network communications may be impaired. RF impairments may occur in many forms including, but not limited to, impulse and/or burst noise, common path distortion, and ingress such as interference from radio communication and navigation signals. Impulse noise or burst noise typically consists of high-power, short-duration energy pulses. Ingress is unwanted RF energy that enters the communication path from a source external to the communication path. Ingress often comprises radio and/or navigational communication signals propagated over the air that enter a weak point in a wireline network, although it may also comprise impulse and/or burst noise that is similarly propagated over the air to enter the network at a weak point. Weak points in the network often occur where there is a cable shield discontinuity, improperly grounded electrical device, or a faulty connector at or near a remote point. When radio frequency carriers from shortwave radio, citizen's band radio, or other broadcast sources enter the network at these weak points, they may cause interference peaks at specific carrier frequencies in the communication path. Common path distortion may be the result of second and higher order mixing products from the downstream channel that couple to the upstream channel; such nonlinear mixing may occur, for example, when physical electromechanical connectors corrode and oxidize, creating point contact diodes.
Therefore, an ability to monitor the performance of the cable network and to quickly and efficiently isolate impairments in the cable network is essential for the cable network operation. Heretofore two main approaches have been used in cable network monitoring: the use of dedicated measurement equipment, either pre-installed throughout the network or portable, by a technician performing a service call, and remote polling of the installed modem base. The remote polling utilizes capabilities of a conventional DOCSIS CM to perform a range of signal measurements on downstream DOCSIS channels used by the CM to determine such performance related parameters as the downstream reception level, upstream transmission level, downstream signal to noise ratio (SNR), downstream code word error (CWE), etc. The headed may collect measured values of these parameters by polling the CMs.
For example, U.S. Patent application No. 2004/0073664 by Bestermann and U.S. Pat. No. 6,393,478 issued to Bahlmann disclose systems in which a cable modem is utilized to transmit status information to another device for analysis over the network. In Bestermann's system, the cable modem includes server software that communicates with client software through the CMTS at the head-end. The modem measures I, Q values on the DOCSIS downstream channel allocated to the modem for data communications, and the server software delivers measurement data stored in a buffer of the cable modem. This communication is performed using the IP protocol, so that no portable diagnostic device needs to be coupled to a CM port. However, these measurements may be insufficient for adequate network monitoring, and additional network measurement equipment may be required to fully characterize the network and isolate network faults. In some cases, a technician with a portable network tester may have to be sent to perform network measurements in the field or at customer premises.
U.S. Patent application No. 2004/0103442 by Eng discloses a system for end-of-line monitoring of a node of a DOCSIS network. The system of Eng includes a CMTS having a status monitoring Media Access Control (MAC), a network manager coupled to the CMTS, and status monitoring cable modems at termination points. The status monitoring cable modem of Eng includes a dedicated measuring device for detecting and measuring downstream communication signals and a transmitter for transmitting status information over a dedicated service channel to the status monitoring MAC at the CMTS. One drawback of the system of Eng is that it lacks the ability to perform measurements in the upstream path, or to monitor non-DOCSIS downstream channels.
A somewhat different approach has been disclosed in U.S. Patent Application 2007/0133425, which is assigned to the assignee of the present application. This application teaches an end-of-line (EOL) monitoring system that utilizes end-of-line monitors and a central network monitoring server. The end-of-line monitor includes a main tuner dedicated to receiving measurement requests from the network monitoring server via a DOCSIS downstream channel, and an auxiliary tuner used for the purpose of performing measurements of the downstream channel by analogue and digital measurement circuits. The addition of the auxiliary tuner permits faster measurements to be performed by the EOL monitoring system and reported back to the server, thereby affording real-time remote testing and diagnostics. This approach, however, may require installing a potentially large number of dedicated end-of-line equipment solely for the purpose of network monitoring.
Accordingly there still remains a clear need for an economical network monitoring system and method that is capable of network-wide monitoring without adding dedicated end-of-line measurement equipment, and which is capable of testing both the downstream and the upstream communication paths. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It is therefore an object of the present invention to provide a system that improves the prior art by overcoming at least some of the disadvantages and limitations thereof by utilizing intrinsic capabilities of DOCSIS® 3.0 compatible cable modems.